For the qualitative or quantitative analytical determination of sample liquids, especially of body fluids, such as blood or urine, in recent times so-called carrier-bound tests have been increasingly used. In these test devices, reagents are embedded in at least one test layer which is brought into contact with the sample. The reaction of sample and reagents gives rise to a detectable signal and especially to a colour change. This can be visually evaluated in simple cases. In the case of quantitative determinations, the evaluation preponderantly takes place with the help of an apparatus and especially reflection photometrically.
Test carriers are frequently made as test strips having a longitudinal support layer of a synthetic resin material to which are supplied one or more test layers. However, other forms of test carriers are known, for example, in the form of quadratic or rectangular platelets.
Test carriers and the associated evaluation devices, which together are referred to as test carrier analysis systems are, in comparison with the previously known wet chemical analysis processes, simple to handle and the apparatus are inexpensive. Consequently, ever more demanding analyses are, to an increasing extent, carried out with the help of test carriers. This has resulted in the development of test carriers which contain several test layers. The test layers are so arranged on a test carrier on top of one another or next to one another that a desired course of the test takes place. By means of appropriate constructional measures, it is even possible to achieve a multi-stage course of a reaction (see Federal Republic of Germany Patent Specification No. 36 38 654).
In particular, the test layers fulfill two functions:
a) Reagent carrier function: Dry reagents are contained in the layer in elutable or carrier-fixed form. Elutable reagents thereby are dissolved or dispersed by the sample liquid and mostly first react after elution into the liquid phase, whereas carrier-fixed reagents participate in the reaction in fixed form.
b) Liquid transport function: The test layers serve for the transport of the sample liquid within the test carrier.
The test layers mostly fulfil both functions simultaneously but sometines there are used solely liquid transport layers (which do not contain reagents) or solely reagent layers (which do not bring about a liquid transport).
The present invention is especially concerned with porous test carrier layers. For the reagent carrier function, the porosity is usually advantageous because of the large surface area involved therewith. At the same time, in general, porous layers fulfill a liquid transport function which is based upon the capillary forces acting in the layer.
Porous test layers have at least one solid component which, alone or with other components, forms a three-dimensional, open-pored structure. The structure can be very different. Thus, for example, test layer papers or porous synthetic resin structures are known which can also be referred to as open-pored membranes or porous films.
Of especial importance for the present invention are textile structures, for example, fabrics or fleece in which the solid component is formed from filaments or threads which are intertwined with one another. Of course, several different materials can thereby also be used which form different solid components.
A further important feature of the present invention are particle-composite structures. Particles, for example, synthetic resin spheroids or inorganic particles thereby form a first solid component. The particles are connected together with an appropriate adhesive, frequently a polymer, to give a three-dimensional open-pored structure. The particles which form the first solid component are collectively referred to as opener particles. Several Different particle materials can be used which form different solid components of the test layer.
The term "solid component" is to be broadly understood to mean that every solid component of a test layer is included, regardless of whether it is itself participating in the structural composition of the test layer or whether it is merely integrated therein as is, for example, the case with the often used particles of titanium dioxide in test layers for reasons of optical reflection.
Very high requirements are demanded of test layers for modern quantitative test carriers. This applies, in particular, to immunochemical processes.
The biochemical reagents necessary for such processes, especially enzymes or enzyme conjugates, must be available for the reaction in the course of the test in very exactly pre-determined amounts. This is in contradistinction to most well-known clinical-chemical processes in which such reagents are usually used in excess without this having a substantial influence on the accuracy of the result. Consequently, immunochemical processes can, as a rule, only be carried out on solid carriers if the necessary biochemical reagents can be applied to a test carrier in a precisely measured amount, if there is no decrease of their enzyme activity even after comparatively long storage and if there is an unchanged dissolving behaviour during the storage up to the time of use.
Test layers for immunochemical tests are also subject to special requirements with regard to the transport properties. Here, it is, in particular, a question of the course of reactions in which the test layer contains an immunological binding component, for example an antibody, in carrier-fixed form and, in the course of a test, a liquid flows therethrough which contains a binding component complementary thereto, for example an antigen. The accuracy of such determinations is, on the one hand, substantially dependent upon the fact that the antigen transported through the test layer binds completely with the carrier-fixed antibody. In addition, however, it is also important that, as far as possible, no non-specific binding takes place. Thus, for example, antigen-antibody complexes formed in a preceding reaction step are to pass as unhindered as possible through the test layer functioning as separation layer, as is explained hereinafter in more detail. In general, in the case of various test processes, it is to be prevented that macromolecular organic test components contained in a liquid flowing through a test layer enter into an undesired binding with any of the solid components of the test layer.